Thermopower and electrical conductivity of sodium-doped V2O5 thin films

Abstract

Thermopower and electrical conductivity of sodium-doped vanadium pentoxide (V2O5) thin films synthesized by a solution technique were investigated. An aqueous solution based V2O5 thin film is a good candidate for thin film devices using ink-jet deposition and screen printing techniques. To improve its thermoelectric properties for practical applications, Na was systematically introduced into V2O5 as a dopant to study how it influences the thermoelectric properties. A melt-quench technique was implemented to dope various concentrations of sodium ions into sol-gel solutions, and thin films were fabricated from these. X-ray diffraction showed that the Na doped V2O5 samples dominantly form the crystalline phase β-NaxV2O5. It was shown that by increasing the Na concentration, the electrical conductivity could be increased by a factor of up to ∼104, whereas the Seebeck coefficient decreased only by a half. Direct measurement of the thermoelectric power output verified that the power factor was improved up to 350 times. Temperature dependent characteristics of the electrical conductivity and the temperature independence of Seebeck coefficient, respectively, in a range of temperatures examined, suggested that the transport mechanism was a small polaron hopping type. A significant increase in the electrical conductivity was ascribed to the increase in the polaron population and the reduction of the activation energy due to formation of β-NaxV2O5.